When an A horizon becomes plowed, it becomes a "plow layer." The physcial properties are modified from the pedogenic original, but pedogenesis proceeds immediately after plowing ceases. Why is it a "layer"? Many people argue whether an older "plow layer" that has reformed granular structure after plowing ceases for an extended time is no longer a "plow layer" but is an "A horizon". Thus there is some confusion between "horizon" and "layer" terminology for the A horizon.
Dr. John Galbraith
Virginia Tech